Carbon is known to have four unique crystalline structures, including diamond, graphite, fullerene and carbon nano-tubes. The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall, which can be conceptually obtained by rolling up a graphene sheet or several graphene sheets to form a concentric hollow structure. A graphene sheet is composed of carbon atoms occupying a two-dimensional hexagonal lattice. Carbon nano-tubes have a diameter on the order of a few nanometers to a few hundred nanometers. Carbon nano-tubes can function as either a conductor or a semiconductor, depending on the rolled shape and the diameter of the tubes. Its longitudinal, hollow structure imparts unique mechanical, electrical and chemical properties to the material. Carbon nano-tubes are believed to have great potential for use in field emission devices, hydrogen fuel storage, rechargeable battery electrodes, and as composite reinforcements.
However, CNTs are extremely expensive due to the low yield and low production rates commonly associated with all of the current CNT preparation processes. The high material costs have significantly hindered the widespread application of CNTs. Rather than trying to discover much lower-cost processes for nano-tubes, we have worked diligently to develop alternative nano-scaled carbon materials that exhibit comparable properties, but can be produced in larger quantities and at much lower costs. This development work has led to the discovery of processes for producing individual nano-scaled graphite planes (individual graphene sheets) and stacks of multiple nano-scaled graphene sheets, which are collectively called “nano-scaled graphene plates (NGPs).” NGPs could provide unique opportunities for solid state scientists to study the structures and properties of nano carbon materials. The structures of these materials may be best visualized by making a longitudinal scission on the single-wall or multi-wall of a nano-tube along its tube axis direction and then flattening up the resulting sheet or plate. Studies on the structure-property relationship in isolated NGPs could provide insight into the properties of a fullerene structure or nano-tube. Furthermore, these nano materials could potentially become cost-effective substitutes for carbon nano-tubes or other types of nano-rods for various scientific and engineering applications. The electronic, thermal and mechanical properties of NGP materials are expected to be comparable to those of carbon nano-tubes; but NGP will be available at much lower costs and in larger quantities.
Direct synthesis of the NGP material had not been possible, although the material had been conceptually conceived and theoretically predicted to be capable of exhibiting many novel and useful properties. Jang and Huang have provided an indirect synthesis approach for preparing NGPs and related materials [B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006)]. In most of the prior art methods for making separated graphene platelets, the process begins with intercalating lamellar graphite flake particles with an expandable intercalation agent (intercalant), followed by thermally expanding the intercalant to exfoliate the flake particles. In some methods, the exfoliated graphite is then subjected to air milling, ball milling, or ultrasonication for further flake separation and size reduction. Conventional intercalation methods and recent attempts to produce exfoliated products or separated platelets are given in the following representative references:    1. J. W. Kraus, et al., “Preparation of Vermiculite Paper,” U.S. Pat. No. 3,434,917 (Mar. 25, 1969).    2. L. C. Olsen, et al., “Process for Expanding Pyrolytic Graphite,” U.S. Pat. No. 3,885,007 (May 20, 1975).    3. A. Hirschvogel, et al., “Method for the Production of Graphite-Hydrogensulfate,” U.S. Pat. No. 4,091,083 (May 23, 1978).    4. T. Kondo, et al., “Process for Producing Flexible Graphite Product,” U.S. Pat. No. 4,244,934 (Jan. 13, 1981).    5. R. A. Greinke, et al., “Intercalation of Graphite,” U.S. Pat. No. 4,895,713 (Jan. 23, 1990).    6. F. Kang, “Method of Manufacturing Flexible Graphite,” U.S. Pat. No. 5,503,717 (Apr. 2, 1996).    7. F. Kang, “Formic Acid-Graphite Intercalation Compound,” U.S. Pat. No. 5,698,088 (Dec. 16, 1997).    8. P. L. Zaleski, et al. “Method for Expanding Lamellar Forms of Graphite and Resultant Product,” U.S. Pat. No. 6,287,694 (Sep. 11, 2001).    9. J. J. Mack, et al., “Chemical Manufacture of Nanostructured Materials,” U.S. Pat. No. 6,872,330 (Mar. 29, 2005).
However, these previously invented methods had a serious drawback. Typically, exfoliation of the intercalated graphite occurred at a temperature in the range of 800° C. to 1,050° C. At such a high temperature, graphite could undergo severe oxidation, resulting in the formation of graphite oxide, which has much lower electrical and thermal conductivities compared with un-oxidized graphite. In our recent studies, we have surprisingly observed that the differences in electrical conductivity between oxidized and non-oxidized graphite could be as high as several orders of magnitude. It may be noted that the approach proposed by Mack, et al. [e.g., Ref. 9, U.S. Pat. No. 6,872,330 and J. J. Mack, et al., “Graphite Nanoplatelet Reinforcement of Electrospun Polyacrylonitrile Nano-fibers,” Adv. Materials, 7 (2005) January 6, pp. 77-80] is also a low temperature process. However, it involves intercalating graphite with potassium melt, which must be carefully conducted in a vacuum or extremely dry glove box environment since pure alkali metals like potassium and sodium are extremely sensitive to moisture and pose an explosion danger. This process is not amenable to mass production of nano-scaled platelets.
To address these issues, we have recently developed several processes for producing nano-scaled platelets, as summarized in several co-pending patent applications [Refs. 10-13]:    10. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Process for Producing Nano-scaled Platelets and Nanocomposites,” U.S. Pat. Pending, Ser. No. 11/509,424 (Aug. 25, 2006).    11. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Mass Production of Nano-scaled Platelets and Products,” U.S. Pat. Pending, Ser. No. 11/526,489 (Sep. 26, 2006).    12. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Method of Producing Nano-scaled Graphene and Inorganic Platelets and Their Nanocomposites,” US Pat. Pending, 11/709,274 (Feb. 22, 2007).    13. Aruna Zhamu, JinJun Shi, Jiusheng Guo, and Bor Z. Jang, “Low-Temperature Method of Producing Nano-scaled Graphene Platelets and Their Nanocomposites,” US Pat. Pending, Ser. No. 11/787,442 (Apr. 17, 2007).
References [10,11] are related to processes that entail a pressurized gas-induced intercalation procedure to obtain a tentatively intercalated layered compound and a heating and/or gas releasing procedure to generate a supersaturation condition for inducing exfoliation of the layered compound. Tentative intercalation implies that the intercalating gas molecules are forced by a high gas pressure to reside tentatively in the interlayer spaces. Once the intercalated material is exposed to a thermal shock, these gas molecules induce a high gas pressure that serves to push apart neighboring layers. Reference [12] is related to a halogen intercalation procedure, followed by a relatively low-temperature exfoliation procedure. No strong acid like sulfuric acid or nitric acid is used in this process (hence, no SO2 or NO2 emission) and halogen can be recycled and re-used. This is an environmentally benign process.
Reference [13] provides a low-temperature method of exfoliating a layered material to produce separated nano-scaled platelets. The method entails exposing a graphite intercalation compound to an exfoliation temperature lower than 650° C. for a duration of time sufficient to at least partially exfoliate the layered graphite without incurring a significant level of oxidation. This is followed by subjecting the partially exfoliated graphite to a mechanical shearing treatment to produce separated platelets. The key feature of this method is the exfoliation at low temperature to avoid oxidation of graphite. This was based on the finding that no oxidation of graphite occurs at 650° C. or lower for a short duration of heat exposure (e.g., shorter than 45 seconds) and at 350° C. or lower for a slightly longer duration of heat exposure (e.g., 2 minutes). The resulting NGPs exhibit very high electrical conductivity, much higher than that of NGPs obtained with exfoliation at higher temperatures.
In all of aforementioned prior art methods and our co-pending applications, the process begins with intercalation of graphite, followed by gas pressure-induced exfoliation of the resulting intercalated graphite. The gas pressure is generated by heating and/or chemical reaction. However, intercalation by a chemical (e.g., an acid) is not desirable. Exfoliation by heat can put graphite at risk of oxidation. After exfoliation, an additional mechanical shear treatment is needed to separate the exfoliated graphite into isolated platelets. In essence, every one of these processes involves three separate steps, which can be tedious and energy-intensive.
It is therefore an object of the present invention to provide a simpler, faster, and less energy-intensive method of expanding a laminar (layered) compound or element, such as graphite and graphite oxide (partially oxidized graphite), to produce exfoliated graphite and graphite oxide and nano-scaled graphite and graphite oxide flakes or platelets.
It is another object of the present invention to provide a convenient method of exfoliating a laminar material to produce nano-scaled platelets (platelets with a thickness smaller than 100 nm and mostly smaller than 10 nm) without the intercalation step and, hence, without the utilization of an intercalant such as sulfuric acid.
It is yet another object of the present invention to provide a convenient method of exfoliating a laminar material to produce nano-scaled platelets without involving a heat-or chemical reaction-induced gas pressurization step.
Another object of the present invention is to provide an effective and safe method of mass-producing nano-scaled platelets.
It is still another object of the present invention to provide a method of producing nano-scaled platelets that can be readily dispersed in a liquid to form a nanocomposite structure.